1. Field of the Invention
The present invention relates to a resin molding, a method for manufacturing the resin molding and a metal mold for manufacturing the resin molding by injection molding method, and more particularly, to a metallic tone resin molding that prevents generation of appearance defects and has a color with metallic feeling caused by bright material to have excellent design properties, and a method and a metal mold for manufacturing the metallic tone resin molding.
2. Description of the Related Art
A four-wheel vehicle provided with a manual transmission (MT) includes a shift lever that is used to select the combination of gears of the MT by a driver. The shift lever is mounted on a floor in a vehicle cabin, an instrument panel (dashboard), a column for supporting a steering wheel, or the like. The shift lever includes a shift knob that is grasped by a driver. The shift knob includes a garnish, such as a shift garnish or a shift knob garnish. The shift garnish has letters or straight lines comprised of a plurality of grooves, that indicate the gear position of the MT. The shift garnish may have a color with metallic tone bright feeling.
An MT shift garnish of the four-wheel vehicle is one of significant factors for determining the impression of a driver's seat, and is an appreciative object under both sunlight and indoor light. Accordingly, a painted-component having a silver metallic color with the bright feeling, has been widely used as the MT shift garnish to obtain high design properties. However, volatile organic compound (VOC) generated due to the painting causes an environmental load, and thus the discharge of the VOC has been regulated in recent years (for example, Air Pollution Control Act of Japan (amended in 2004)). For this reason, there is a demand for a shift garnish that is formed without painting.
An injection molding method using resin material which colored with a colorant such as pigment or dye is known as such a paintless method. To color the resin material with a silver metallic color having high bright feeling, bright material such as aluminum powder or mica powder is added to the resin material. Particularly, to obtain high metallic feeling or pearl feeling, it is necessary to add the bright material.
However, appearance defects such as a “weld line” or a “flow mark” are likely to occur when using such resin material to which the bright material is added. In molding, a collision or a turbulent flow of the resin material occurs in the metal mold and thus flowability of the resin material deteriorates. Accordingly, the bright material is likely to be orientated in the metal mold. This orientation results in the weld line or the flow mark. In particular, the appearance defects such as the weld line or the flow mark are likely to occur when rectangular grooves are formed on a design surface of an MT shift garnish. As shown in FIG. 10A, the design surface of the MT shift garnish is formed with the rectangular grooves which indicate operative positions and directions of a shift lever. Therefore, unlike the painting, it is very hard to maintain high design properties in the resin molding. That is, it is very hard to secure the silver metallic tone which has high quality feeling while preventing the generation of the appearance defects. Further, the appearance defects such as the weld line or the flow mark are also likely to occur when the resin material contains bright material having high aspect ratio since the flowability of the resin material is decreased due to the bright material.
Patent Document 1 discloses a method for mixing 0.1 to 20.0 parts by weight of a bright material which has a maximum outer diameter in the range of 10 [μm] to 1 [mm] to 100 parts by weight of a thermoplastic resin so that a mean particle spacing D of the bright material and a weld width H satisfy an expression D≧H. This method disclosed in Patent Document 1 can suppress the weld line, but cannot completely eliminate the weld line. Further, this method cannot be applied to bright material that has a maximum outer diameter smaller than 10 [μm].
Patent Document 2 discloses a method for eliminating the weld line by adding a titanium oxide, a lead oxide, or a zinc oxide, which is called a weld-eliminating agent, to a resin. However, this method disclosed in Patent Document 2 increases cost and causes difficulty in color matching. Although Patent Document 2 mentions a high quality color tone such as a metallic tone or a pearl tone, can be obtained by the disclosed method, the metallic feeling significantly deteriorates when the weld-eliminating agent is added in actuality.
Patent Document 3 discloses a method for adding 0.05 to 10 parts by weight of metal powder that has the aspect ratio in the range of 3 to 15 and the mean particle diameter in the range of 10 to 300 [μm], and adding predetermined parts by weight of a thermoplastic resin (for example, polyethylene or polypropylene) that has a reactive group other than a polyamide resin, which suppresses a weld line, in order to obtain a metallic tone polyamide resin molding which has no weld line and excellent appearance without painting the resin molding. However, Patent Document 3 is completely silent about a molding formed with rectangular grooves on a design surface thereof. Further, Patent Document 3 does not discuss the degree of the metallic feeling.